1. Field of invention
The invention relates to a data processing system wherein a host unit is effectively connected or disconnected during the execution of a command depending upon the command and address.
2. Description of Related Art
FIGS. 10, 11 and 12 show prior technologies. FIG. 10 shows a standard connection between a host unit 131 and plural input/output (I/O) devices 141, 143 and 145. Plural control units 133, 135 and 137 are serially connected to the host unit 131, and a terminator (T) 139 is connected to the control unit 137. The I/O device is connected to each control unit. Lines 132, 134, 136 and 138 are parallel data busses. The host unit 131 includes a host processor, an operating system and a host channel. The host channel decodes a channel command word (CCW) from the host processor to supply a command and I/O device address to the control units connected in series. The control unit decodes the command to control the addressed I/O device. These operations have been called channel operations, and are well known in the art. One of the conditions required for the channel operation is that a response period in an initial sequence, from the time at which the host unit sends the command and the I/O device address to the time at which the control unit returns the response, must be shorter than a predetermined time period, such as 32 micro seconds. If none of the control units is assigned the I/O device address sent from the host channel, the command and the I/O device address are sent to the terminator (T) 139 and returned to the host channel. This must be done in a time period shorter than 32 micro seconds. If the response time is longer than 32 micro seconds, the initial selection sequence is deemed as an error, and the channel operation can not proceed. Therefore, the total length of the lines 132, 134, 136 and 138 has been selected to satisfy the above condition. In the past, the length has been limited to about 400 feet. However, in many instances, it is required to locate the control unit at a remote location over the 400 feet length limit. To solve this problem, the systems shown in the FIGS. 11 and 12 have been proposed. In the FIG. 11, the host unit 146 and a channel-to-channel adapter 147 are located at a local location, and the channel-to-channel adapter 149, the host unit 150, the control unit 151 and the I/O device 152 are located at a remote location, and a long transmission line 148 connects them. This approach locates the control unit 151 at the remote location, but requires the additional host unit 150 and an operator for operating it. This system has been installed in customer locations, but has the disadvantage of increasing of total cost due to the mentioned additional host unit 150 and the operator.
FIG. 12 shows a system disclosed in Japanese patent 1411818 wherein a channel adapter 155 and I/O adapter 157 are connected between the host unit 153 and the I/O device 159. Lines 154 and 158 are parallel data transmission lines and line 156 is a serial data transmission line. The channel adapter 155 and the I/O adapter 157 operate as buffers between the host unit 153 and the I/O device 159. In the configuration of FIG. 12, the connection between the host unit 153 and the channel adapter 155 is maintained during the process of the command, so that other jobs of the host unit must wait for the completion of the command.